Cloudflare Outage on December 5, 2025: Technical Summary

On December 5, 2025, Cloudflare experienced a network-wide outage lasting approximately 25 minutes, affecting around 28% of all HTTP traffic served by the company. The incident was not caused by a cyber attack or malicious activity, but rather by changes made to Cloudflare's body parsing logic while attempting to detect and mitigate an industry-wide vulnerability in React Server Components. The issue was triggered by an increase in buffer size from 128KB to 1MB, which was being rolled out using Cloudflare's gradual deployment system.

Key Technical Details

• The outage was caused by a bug in Cloudflare's FL1 proxy, which resulted in HTTP 500 error codes being served from the network.
• The bug was triggered by a change to turn off Cloudflare's internal WAF testing tool, which was implemented using the global configuration system.
• The issue was identified shortly after the change was applied, and was reverted at 09:12, after which all traffic was served correctly.
• Customers that had their web assets served by the older FL1 proxy and had the Cloudflare Managed Ruleset deployed were impacted.

Practical Implications for Developers

• Developers who use Cloudflare's services should be aware of the potential for outages and plan accordingly.
• The outage highlights the importance of testing and validation of changes before deploying them to production.
• Developers should also be aware of the potential for errors in Cloudflare's rulesets system and take steps to mitigate them.
• The incident also underscores the need for companies to have robust incident response plans in place to minimize the impact of outages.